What causes drift in Thermocouples?
11 January 2018 By Robert Godley
What causes drift in Thermocouples? Is there a solution?
It goes without saying temperature measurement is one of the key factors when working in the heat treatment industry. However, the type K and N base metal thermocouple sensors used today to conduct these measurements have seen little change or improvements in the development in their performance since their initial development (110 and 50 years ago respectively).
With the development of the new low drift high temperature mineral insulated base metal thermocouples I decided to take a look into the workings of type K and N base metal thermocouples and why they drift.
Let’s break it down... so, what actually is a thermocouple?
A thermocouple is a temperature-measuring device consisting of two dissimilar conductors or wires that are joined each other at one end. The joined end is referred as the Hot Junction. The other end of these wires or dissimilar metals are referred to as the Cold Junction.
Thermocouple drift plain and simply from use. The use of a thermocouple in harsh, high temperature environment change the physical state of the material.
The main causes of this are;
- - Oxidisation
- - Metallurgical changes to the thermal element
- - Vaporisation
- - Contamination from the atmosphere and protective outer sheath
These result in the original starting alloy or dissimilar metals of the element to slightly change to essentially different alloys.
Now we’ve established what causes drift in type K and N base metal thermocouple sensors, let’s look into solving the problem.
Over 10 years ago a University student, Michele Scervini, studying at the University of Cambridge started a project looking at temperature measurement using contact sensors.
He conducted his research on the drift characteristics on base metal thermocouples and studied the reason why base metal thermocouples drift.
The conclusion of the study was the major issue of drift in base metal mineral insulated thermocouples was caused by the outer sheath, something that could not be altered due to the properties of the sheath as it was essential to the high temperature application it was used in.
A second inner sheath was developed, creating a barrier between the outer sheath and the conductors to reduce migration, improve temperature measurement accuracy, extend thermocouple life by significantly reducing the drift characteristics.
The new cable then underwent many continuous days and months of rigorous testing at the CCPI Europe UKAS accredited (no. 0600) thermal calibration laboratory and University of Cambridge laboratory. Compared with a standard issue type K and N base metal mineral insulated thermocouple, the drift was reduced by over 80% at 1200 ℃ (2192 ℉) and 90% at 1300 ℃ (2372 ℉).
In industries where the accuracy of the heat treatment is fundamental, the new low drift cable allows the user to know the reading is accurate and reliable!
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